Facilities producing electrical power or steam conventionally burn hydrocarbons, e.g., oil, gas, or coal, in a process that produces substantial emission of CO2 greenhouse gases, particulates, and hazardous air pollutants (HAP's). Electrical power plant emissions are practically unavoidable in a conventional burner/boiler system, and control of these materials requires multiple, expensive, post-combustion treatment systems that capture fly ash, SOx, NOx and trace metals. Economical methods for capturing and sequestering the CO2 do not exist, and accordingly such power plants contribute significantly to greenhouse gases. More currently available methods for combustion treatment of gases either reduce the fuel efficiency of the power plant, e.g., by preventing complete combustion, or significantly add to the operating cost of the system, or both.
Further, economic methods for capturing and sequestering the CO2 do not exist, and accordingly such power plants contribute significantly to the concentration of greenhouse gases in the atmosphere.
In conventional boilers, the rapidly expanding combustion gas is discharged from the hot zone. In coal burning systems, for example, exit velocities are sufficient to pneumatically convey fine particles out of the combustion zone, including ash with unburned carbon. As much as 2% of the input fuel value can be lost in this manner. Conventional systems also operate with high temperatures to promote complete combustion of the fuel in a relatively short period of time if the fuel is in the combustion zone, which is typically milliseconds to seconds in length. Higher combustion temperatures promote higher levels of NOx and SO2 and accelerate degradation of the burner zone. Most fuel-burning systems transfer heat or combustion to a working fluid, such as water or steam, which then drives a turbine.
Not all heat is extracted from the combustion gas, however. The temperature and pressure of the gas exiting the exchanger must be sufficiently high to both drive the exhaust gas up the stack and to assure that the temperature within the stack is above the dewpoint. Water vapor is a combustion by-product and condensation of that vapor within the exhaust system is deleterious. Accordingly, the exhaust gas stream transfers substantial heat from the power plant to the atmosphere; as much as 5% to 15% of the incoming fuel value is lost to the atmosphere in this manner.
Partly to counteract this phenomenon, conventional systems operate at high temperatures to promote more complete combustion of the fuel in the relatively short period of time that the fuel is in the combustion zone, typically milliseconds to seconds in length. An undesirable consequence of this adjustment is higher levels of NOx and SOx in the gas stream, which in turn accelerates degradation of the metal structures in the burner zone and forces installation of stack scrubbers to reduce the NOx and SOx concentration in gases discharged to the atmosphere.
Multiple systems are available for post-combustion treatment of gas streams to remove NOx and SOx by circulating the exhaust gas through solid or liquid reactants. These systems consume more energy than they generate, effectively derating the power plant. The chemical reactants employed provide no significant function beyond capture of NOx and SOx and their cost serves to reduce plant profit margins.
Systems for the economic removal of other pollutants from conventional fuel combustion processes, such as mercury, radionuclides and other metals, are practically not available. Almost all solid and liquid hydrocarbon fuels contain trace amounts of mercury and radioactive elements. Industrial by-products that are combusted in waste-to-energy systems also contain metals, including in some cases precious metals. While metal concentration in the exhaust gas stream is low and daily emissions minimal, over time these pollutants accumulate to potentially harmful concentrations in downwind soil and bodies of water. In the case of precious metals, current combustion systems cannot be configured for their economic recovery.
In the case of mercury and other high vapor pressure metals, capturing these pollutant from the gas phase requires a substantial expense for cooling to condense or adsorb the metal on a suitable collection device. The cooling system would be an energy drain, further derating the plant. While a cooling process might recover waste heat, this approach is not considered economically viable.
While condensation might effect removal of mercury, radioactive particles can only be removed by physical separation, e.g. filtration, or by chemical reaction. Due to the large volume of gas and water vapor produced by combustion and the low concentration of radioactive particles, these approaches are not economically viable. As a result, the radioactive material is left to accumulate downwind from a conventional power plant.
Of all of the emissions from power plants, the one of greatest current concern and research is carbon dioxide. Numerous activities are underway worldwide to sequester emitted CO2 in an effort to reduce its concentration in the atmosphere.
Several proposed methods separate CO2 from the exhaust gas and inject it into stable geologic or marine environments, physically isolating it from the atmosphere. The stability and duration of that separation is not well understood, and over time this stored gas will likely migrate to the atmosphere. A beneficial use of this method is injection for enhanced oil recovery (EOR), where the CO2 both increases formation pressure and lowers the viscosity of the oil. In both geologic and marine applications, substantial energy is expended to separate the CO2 and then pump it to the depths required.
Other proposed methods use the separated CO2 as a reactant in a variety of post-combustion chemical synthesis reactions. For instance, the CO2 can be bubbled through aqueous solutions or slurries containing metal oxides or metal ions. By adjusting reactant concentration and the mixture's temperature, a solid mineral particle may be precipitated. These precipitates may be collected and dried, then disposed or used as an input to another product. These systems largely depend on endothermic reactions, requiring that power be diverted from the plant. Biological methods are also proposed, utilizing both the ecosystem (ocean waters, if proximate to the power plant) and controlled environments (e.g., bioreactors). Again, there is an energy cost for separation and pumping, plus the additional capital and operating expenses for complementary nutrients and chemistry.
All of these systems share the common disadvantages of attempting to separate and utilize CO2 in post-combustion devices. Post-combustion processing increases the cost and complexity of building and operating a power plant, as well as reduces the net energy output of the plant.
Some proposed systems, such as the Integrated Pollutant Removal (IPR) system co-developed by DOE Albany Research Center and Jupiter Oxygen Corp., separate oxygen from air prior to combustion, then feed oxygen only into the combustion chamber. The resulting exhaust gas stream is largely CO2, minimizing or eliminating the requirement for post-combustion gas separation. Further development of the IPR system is directed to filtering and compressing the exhaust gas to remove particulates, recover additional energy that might otherwise be discharged through the stack and condition the CO2 for commercial uses or geologic injection. Gas compression, however, is a substantial additional energy and capital cost for this system.
U.S. Pat. No. 6,372,156 discloses a method of chemically converting raw material to another material utilizing a hybrid plasma system. The system utilizes a plasma including activated hydrogen and oxygen formed from a water vapor. U.S. Pat. No. 5,125,965 discloses a process for enhancing fluidization in a fluidized bed reaction chamber. In a preferred embodiment, the molybdenum oxide is reduced to a molybdenum metal. U.S. Pat. No. 4,368,169 discloses a pyrochemical process for the decomposition of water. The process is carried out in a reaction chamber of a reactor during and immediately after a thermonuclear reaction. The initial chamber reaction yields a condensed metal oxide product which is split in a later process to yield oxygen and a condensed metal product.
The disadvantages of the prior art are overcome by the present invention, and the included system is hereinafter disclosed for combusting carbon-containing fuel to minimize pollution from fuel combustion.